In general, when a boat first accelerates from a standstill or a relatively slow speed before the boat gets up on plane, the bow of the boat lifts out of the water. Bow lift (i.e., hull pitch attitude for the boat) can become steep when accelerating smaller, shorter boats rapidly from a standstill or a very slow speed. Boat acceleration and steering control are compromised when the hull pitch attitude becomes excessively steep. Therefore, it is advantageous to keep the bow of a boat relatively low, not only to improve boat acceleration and steering control, but also to improve driver visibility. Bow lift during slow speed acceleration can be reduced by trimming the propeller inwardly towards the transom of the boat.
Normally, marine propulsion systems, e.g., outboard motors and inboard/outboard motors (stern drives), are trimmed to orient the propeller in a position that optimizes performance after the boat gets up on plane. Trimming the marine propulsion system to orient the propeller in a position which reduces bow attitude while accelerating at slow speeds tends to compromise performance when the boat gets up on plane. Therefore, it is desirable to provide a trim system that automatically adjusts the trim angle of the marine propulsion system inwardly towards the boat in response to increased loads on the propeller.